Polyalkylene terephthalates such as polyethylene terephthalate (“2GT”) are common commercial polyesters. They have excellent physical and chemical properties, including chemical, heat and light stability, high melting point and high strength. As a result they have been widely used for resins, films, and fibers.
A key difference between drawing nylon and PET fiber lies in the temperature to which the undrawn yarn is raised to allow the fibers to start drawing in a uniform manner with a reasonable draw force. Nylon and PET can be drawn at room temperature but are best drawn at a temperature above their glass transition temperatures of about 40° C. and 65° C. respectively to obtain uniform physical properties and/or preclude undue filament breakage during drawing. The glass transition temperature (Tg), also called the second order transition temperature, can be obtained by dilatometric methods. The undrawn yarn can be raised to above its Tg before drawing with draw assists such as heated rolls.
The making of polyester and nylon staple fibers often involves a multi-stage process. In the first step, polymer is extruded into filaments, which are quenched, attenuated, and lubricated; and the filaments of each spin position are combined into a filament bundle. Filament bundles from the individual spin positions are then immediately combined at the spinning wall into a spun rope. Drawing of the spun rope to form an oriented structure having useful properties is often done in a separate step, wherein the spun rope is piddled into a can for subsequent drawing and texturing. The spun cans are assembled into a creel of economic size, for drawing at the drawing machine. In this split spin/draw staple process, there is an inherent time delay between the extrusion and drawing process to allow for producing such a creel for drawing. This delay is often substantial and depends, in part, on the number of spin positions and spin rate of the spinning machine. Further, production schedules can extend the delay before drawing to days rather than hours.
After the fiber is drawn to give it adequate strength for downstream processing and end use, it is textured and lubricated to provide appropriate fiber friction and value. A stuffing box crimper usually carries out the nylon and PET staple texturing. The crimping equipment and process conditions can impact the type, frequency, and permanence of the crimp. The crimped tow can be pre- or post treated with lubricants, dried, relaxed or annealed, and cut into staple fibers and baled. The operations from drawing to baling can be carried out in separate steps or in a coupled process. The optimum conditions depend on the fiber composition and end use, and the cut length depends on end use and staple processing system i.e. cotton, wool, modified worsted. The cotton system equipment generally uses short fibers (1-3 inches) for textile applications and the modified worsted system, used for carpet processing, uses longer fibers (6-8 inches).
Bales of the cut staple fibers are converted into a continuous yarn in a multi-stage mill operation-using opening, blending, carding, drafting, and spinning equipment. Certain physical properties are highly desirable in the fibers so that they can undergo the drawing and texturing processes without diminished quality in the resulting fiber. One of the most critical parameters is crimp frequency (crimps per inch, c.p.i) and its permanence (crimp take-up, CTU). It is desirable that the staple fibers have enough crimp to provide adequate sliver cohesion but not too much to cause excessive fiber entanglement in operations such as blending. The crimp should be permanent enough to withstand the considerable forces in mill processing. For example, when such fibers are carded to comb them to parallelism, they can, because of entanglement, be snarled into defects or stretched until crimp is permanently removed or the filaments break. Also if the crimp is lost, either from stretching or due to insufficient permanence, the sliver leaving the card can have insufficient strength and cohesion and could break and prevent further operation. Even though the CTU is increased with crimp frequency, it is desired that the fiber have a balance of crimp frequency and CTU to prevent excessive entanglement from too high crimp. Carpet fibers have higher denier than textile fibers and are stiffer so they require lower crimp levels to prevent entanglement. In addition, any crimp loss reduces the bulk of the yarn, which reduces the value of the carpet. Lower yarn bulk provides less cover and so requires more weight for equal cover. Processing lubricants are applied to help control fiber-to-fiber and fiber-to-metal friction, and provide static protection. In carpet production, the spun yarns are typically plied, heatset to set the twist, tufted into a primary backing, and dyed. Then, a secondary backing is applied to the primary backing, using a latex adhesive, which locks in the tufts and provides dimensional stability to the carpet.
Poly(trimethylene terephthalate), also referred to as PTT or 3GT is a polyester suitable for use in carpet, textile, and other thermoplastic resin applications. Poly(trimethylene terephthalate) in fiber form is desired because it can be dyed at atmospheric pressure with disperse dyes, has a relatively low bending modulus, a relatively high elastic recovery and resilience, and resistance to staining. However, undrawn PTT yarns, under some spinning conditions can become brittle upon aging (e.g., storage). Conventional two-step processes used for making polyester staple fibers, as mentioned hereinabove, include an inherent time delay between the extrusion and drawing process, which effectively ages the fibers. Brittle fibers can be difficult to draw and may even be undrawable.
U.S. Pat. No. 6,109,015 discloses an attempt to overcome the problem of brittleness in PTT. The patent discloses a continuous process for producing PTT yarn stated to have improved wear over yarns made in conventional two stage processes. The continuous process avoids the aging of the fiber by eliminating the storage step by coupling the spin and draw steps. However, the process also requires major equipment modifications, which prevents the use of existing conventional two-step equipment.
Other efforts to overcome problems associated with aging of undrawn yarns were directed to reduction or control of shrinkage. For example, Patent Publication No. WO 01/68962 A2 discloses a two-step process for producing fine denier textile yarns from poly(trimethyelene terephthalate) on equipment with relatively long quench zones. The first step produces undrawn yarn, and the second step converts the undrawn yarn to a staple fiber. The process includes preconditioning the fiber under tension at a temperature of 60° C. or above, then drawing the fiber, at a temperature of 60° C. or above, preferably to 80-85% of the total draw length of the fiber. After an optional second drawing stage, the fiber is relaxed at a temperature of up to 190° C.
In certain textile end uses, staple fibers are preferred over continuous filament. Examples include staple spun yarns for apparel fabrics (1-6 dpf) and carpet (6-25 dpf) both of which require discontinuous fiber rather than continuous to permit use of textile staple processing equipment. The manufacture of staple fiber suitable for fabrics and carpets can present special problems, particularly in conventional split spin/draw processes where the drawing is carried out in a separate step. A need thus remains for processes for manufacturing fiber, and particularly staple fiber, from PTT.